The tubulysins are members of a new class of natural products isolated from myxobacterial species (F. Sasse, et al., J. Antibiot. 2000, 53, 879-885). As cytoskeleton interacting agents, the tubulysins are mitotic poisons that inhibit tubulin polymerization and lead to cell cycle arrest and apoptosis (H. Steinmetz, et al., Chem. Int. Ed. 2004, 43, 4888-4892; M. Khalil, et al., ChemBioChem. 2006, 7, 678-683; G. Kaur, et al., Biochem. J. 2006, 396, 235-242). Tubulysins are extremely potent cytotoxic molecules, exceeding the cell growth inhibition of any clinically relevant traditional chemotherapeutic e.g. epothilones, paclitaxel, and vinblastine. Furthermore, they are potent against multidrug resistant cell lines (A. Dömling, et al., Mol. Diversity 2005, 9, 141-147). These compounds show high cytotoxicity tested against a panel of cancer cell lines with IC50 values in the low picomolar range; thus, they are of interest as potential anticancer therapeutics.
Tubulysins are described herein. Structurally, tubulysins often include linear tetrapeptoid backbones, including illustrative compounds having formula T

and pharmaceutically acceptable salts thereof;
wherein
Ar1 is optionally substituted aryl;
R1 is hydrogen, alkyl, arylalkyl or a pro-drug forming group;
R2 is selected from the group consisting of optionally substituted alkyl and optionally substituted cycloalkyl;
R4 is optionally substituted alkyl or optionally substituted cycloalkyl;
R3 is optionally substituted alkyl;
R5 and R6 are each independently selected from the group consisting of optionally substituted alkyl and optionally substituted cycloalkyl;
R7 is optionally substituted alkyl; and
n is 1, 2, 3, or 4.
Another illustrative group of tubulysins described herein are more particularly comprised of one or more non-naturally occurring or hydrophobic amino acid segments, such as N-methyl pipecolic acid (Mep), isoleucine (Ile),
and analogs and derivative of each of the foregoing. A common feature in the molecular architecture of the more potent natural occurring tubulysins is the acid and/or base sensitive N-acyloxymethyl substituent (or a N, O-acetal of formaldehyde) represented by R2-C(O) in the formula (T).
Another illustrative group of tubulysins described herein are those having formula 1.
